Conventionally, a metal oxide, such as antimony-doped tin oxide (ATO), and tin-doped indium oxide (ITO), has been used as a transparent electroconductive film for an electrode of a display element (e.g., a liquid crystal display element, and an electroluminescence display element), or a heat element for preventing a windscreen or window glass of cars, aircrafts, or buildings from clouding or freezing.
It has been recently discovered that an oxide semiconductor, such as ZnO, In2O3, and In—Ga—Zn—O, which is a type of the metal oxide, is a semiconductor exhibiting high carrier mobility compared to amorphous silicon. Moreover, a development of a field-effective transistor (FET) using such oxide semiconductor for an active layer has been actively conducted.
As for a method for forming a thin film of such metal oxide, vacuum deposition, and sputtering are common. For example, disclosed is a thin semiconductor film, which is formed using a vacuum film forming technique, such as sputtering, contains indium, a positive dyad (particularly, zinc, magnesium, copper, cobalt, nickel, and calcium), and oxygen, and has specific resistance of 10−1 Ωcm to 108 Ωcm (see, for example, PTL 1).
In order to perform this problem, however, it is necessary to prepare a device that is complex and expensive. Moreover, it is difficult to form a thin film of a large area with this method.
As for a method enabling to form a thin film of a large area more simply, therefore, it has been actively developed a metal (semiconductor) nano particle ink, which is prepared by forming metal or semiconductor particles, which are called nano particles each having a diameter of 1 μm or smaller, in a vapor phase or a liquid phase, and dispersing the nano particles in a solvent. However, the metal (semiconductor) nano particle ink cannot avoid separation, aggregation, and segmentation in an ink tank or a flow channel, and it is difficult to secure uniform dispersibility, and storage stability thereof. Therefore, it is difficult to use the metal (semiconductor) nano particle ink over a long period of time. In order to apply the ink, in which metal or alloy nano particles are dispersed, and oxidize the nano particles to the insides thereof through baking, it is necessary to perform baking at high temperature. Therefore, it is difficult to make a composition ratio between the metal element and oxygen uniform. In the case where an ink, in which oxide nano particles are dispersed, is used, moreover, it is difficult to produce nano particles having uniform particle diameters, and to reduce contact resistance between nano particles during baking.
Meanwhile, considered is a coating method using a coating liquid, in which an inorganic metal compound, or an organic metal compound is dissolved in an organic solvent, and other metals are added as an active agent to impart higher electroconductivity. In this case, the compound contained is uniformly dissolved in the solvent, and therefore uneven distribution of a concentration in the coating liquid, or separation in the coating liquid is hardly caused, and the coating liquid can be used over a long period of time. Moreover, a thin film produced using this coating liquid has a uniform composition, and therefore it has excellent uniformity in properties when the thin film is used as an active layer of a field-effect transistor.
For example, for the purpose of forming a thin film having high electroconductivity and transmittance, disclosed is a composition for forming a transparent electroconductive film, which contains an inorganic indium compound, a magnesium compound, and an organic compound that can be coordinated to the indium (see, for example, PTL 2). Moreover, disclosed is a composition for forming a transparent electroconductive film, in which indium nitrate, a condensate of polyhydric alcohol, and an active agent are dissolved in an organic solvent (see, for example, PTL 3).
These disclosed methods are however methods associated with a composition for forming a transparent electroconductive film, and an obtainable transparent electroconductive film does not have sufficient functions as an active layer of a field-effect transistor. Therefore, use of such film is limited.
Furthermore, disclosed are a solution of a metal oxide precursor, in which a metal oxide precursor is an organic metal salt, and the inorganic metal salt is dissolved in a solvent that is water or ethanol, and a method containing applying the solution of a metal oxide precursor onto a base to form oxide semiconductor (see, for example, PTL 4). In this literature, use of the obtained oxide semiconductor for an active layer of a field-effect transistor is discussed.
However, the descriptions of the metal salt in PTL 4 merely list metals excluding elements that are gas at normal temperature and pressure, noble metals, halogens, Group 16 elements, P and As belonging to the Group 15, and radioactive elements, and also describes only that, among the aforementioned metals, it is preferred that at least one selected from the group consisting of indium, tin, and zinc be contained. In accordance with the disclosed method, moreover, the solution of a metal oxide precursor is thinly spread over the base once the solution is applied onto the base, and therefore accuracy of a shape of the obtained oxide semiconductor is low.
Furthermore, researched is, in a case where a film of a desired shape is directly formed by a printing process, such as inkjet printing, nano imprinting, and gravure printing, a system where printing is performed in a roll-to-roll processing using a plastic substrate, such as polycarbonate (PC), polyimide (PI), polyethylene terephthalate (PET), and polyethylene naphthalate (PEN), or a thin plate glass having a thickness of 50 μm to 70 μm. In this case, reduction in the process temperature is strongly desired for considering thermal resistance, or reducing thermal elongation of the substrate.
Accordingly, there is currently a need for a coating liquid for forming a metal oxide film, with which a metal oxide film having desired volume resisitivity can be produced easily at low process temperature and into a large area, and with which a desired shape of a metal oxide film can be formed with high accuracy.